FAQ: Iterators - Introduction to Iterators

I have a question concerning a return value in the lesson ‘Introduction to iterators’ as following:

const things = ['desk', 'chair', 5, 'backpack', 3.14, 100];

const onlyNumbers = things.filter(thing => {
  return typeof thing === 'number';
});

I would suggest that the returning value would evaluate to be a Boolean, though the parameter string is being returned as value.

I thought this is some special rule that is preprogrammed. Does anyone have more info about this?

Thanks in advance.